1. Field of the Invention
The present invention relates to an oil equalizing system for multiple compressors used in an air conditioner or the like which is capable of maintaining a proper amount of oil in each compressor.
2. Description of the Related Art
As an example of one type of air conditioner, there is a so-called “multi-type air conditioner in which a plurality of compressors are provided in one outdoor unit, in order to cope with a plurality of indoor units.
For the multiple compressors provided in the outdoor unit of such an air conditioner, variable capacity compressors may be used. In some cases, such compressors may have different capacities of compressor shells thereof.
In such cases, where compressors communicate via an oil equalization tube, oil may flow from the shell of the high pressure side compressor to the low pressure side compressor. In this case, the oil flows continuously, even when the level thereof in the shell of the high pressure side compressor is lowered below the position of oil equalizing tube connectors. This is because the oil is present in a mist state as it is stirred by rotating elements in the shell of the high pressure side compressor. As a result, shortage of oil in the high pressure side compressor may occur.
In order to prevent such a flow of oil mist, a proposal has been made, in which the shells of multiple compressors are communicated via an oil equalizing tube, and the oil equalizing tube is connected to a discharge side refrigerant line of the compressors via a bypass tube, as disclosed in Japanese Laid-open Publication No. Heisei 04-222354.
The oil equalizing system for multiple compressors disclosed in the above publication will be described in brief. As shown in FIG. 2, in a refrigerant circuit Ka, three compressors 1, 2 and 3 are connected to a discharge side refrigerant line 5, and a suction side refrigerant line 6, such that the compressors are connected in parallel. Respective compressors 1, 2 and 3 include shells 1a, 2a and 3a, adjacent ones of which are communicated via an oil equalizing tube 7. The discharge side refrigerant line 5 of the compressors 1, 2 and 3 is connected to the oil equalizing tubes 7 via a bypass tube 9, which is provided with an opening/closing valve 8 at an intermediate portion thereof.
In accordance with this oil equalizing system, the opening/closing valve 8 is open during normal cooling/heating operation so that high pressure refrigerant gas is introduced into the oil equalizing tubes 7 via the bypass tube 9. Accordingly, it is possible to prevent oil mist from flowing between adjacent compressor shells 1a, 2a and 3a through the associated oil equalizing tube 7, and thus, to prevent shortage of oil in the high pressure compressor.
Where there is an oil amount difference among the compressor shells 1a, 2a and 3a due to a prolonged compressor operation, a so-called “oil equalizing operation” is carried out. That is, the compressors 1, 2 and 3 are sequentially operated one by one with the opening/closing valve 8 closed, thereby causing surplus oil in each of the compressors 1, 2 and 3 to be sequentially fed. Thus, the amount of oil in each of the compressor shells 1a, 2a and 3a is returned to a proper value.
However, the conventional oil equalizing system for multiple compressors shown in FIG. 2 has various problems.
That is, the shell of the intermediate one of the three compressors 1, 2 and 3, that is, the compressor shell 2a, communicates with the shells 1a and 3a of the left and right compressors 1 and 3 via respective oil equalizing tubes 7, so that it is necessary to use two oil equalizing tube connectors. For this reason, a particular machining process is required for the compressor shell 2a, so that there is an increase in costs.
Furthermore, when the level of oil in the compressor 2 arranged at the middle side as viewed in FIG. 2 is lowered below the level of the oil equalizing tube connectors 10 while the compressor 1 arranged at the left side as viewed in FIG. 2 operates in the oil equalizing operation mode, in which the multiple compressors are sequentially operated one by one, only the refrigerant, which is introduced into the compressor 2 from the suction side refrigerant tube 6 connected to the compressor 2, is fed to the compressor 1, which is in operation, via the associated oil equalizing tube 7 (as indicated by a white arrow in FIG. 2). In this state, the oil of the compressor 3, which is arranged at the right side as viewed in FIG. 2, cannot reach the compressor 1, which is in operation. For this reason, there is a problem in that it is impossible to return the amount of oil in each of the compressor shells 1a, 2a and 3a to a proper value, even through the oil equalizing operation is carried out.
Meanwhile, although the left and right oil equalizing tubes 7 communicate by the bypass tube 9, no liquid oil can flow between the compressors via the bypass tube 9 because the bypass tube 9 has a diameter considerably smaller than that of the oil equalizing tubes 7.